Generally, a printed wiring board is a board having conductor layers 4 and 5 made of copper foil on both sides of an insulation layer 1 formed by impregnating glass cloth 3 with resin 2 and hardening, as shown in FIG. 9; a printed wiring board of a board shape made up of multiple layers as shown in FIG. 10 is also available.
Hitherto, the following two methods for forming a blind hole to electrically connect the conductor layers 4 and 5 on both sides of the insulation layer 1 in such a printed wiring board have been available:
The first method is a method using carbon dioxide laser light, wherein the conductor layer 4 on the laser light incidence side is removed by any other method than the carbon dioxide laser light, such as etching or drilling, and then only the insulation layer is machined by applying laser light using the fact that the carbon dioxide laser light is almost reflected on the conductor layer although it is well absorbed in the insulation layer 1.
The second method is a method using solid state (YAG, etc.,) laser light, wherein a blind hole is formed only by applying laser light using the fact that the solid state laser light is well absorbed in both the insulation layer and the conductor layer unlike the carbon dioxide laser light.
However, to remove the conductor layer by drilling in the first method, it is difficult to make fine adjustment in the depth direction and it is impossible to stably remove the conductor layer so as not to cause damage to the conductor layer 5 on the bottom. To remove the conductor layer by etching, there is a problem of increasing the cost because the etching process is complicated.
Further, to remove the conductor layer 4 and the insulation layer 1 by applying solid state (YAG, etc.,) laser light in the second method, there is a problem of increasing the production cost because the solid state laser running cost is high.
Considering the problems, at present both the conductor layer 4 and the insulation layer 1 are machined to form a blind hole only by applying the carbon dioxide laser light.
Specifically, to machine the conductor layer based on the carbon dioxide laser light, reflection of the carbon dioxide laser light on the conductor layer on the laser light incidence side is remarkable large and thus energy of the carbon dioxide laser light applied to stably machine the conductor layer is made fairly large as compared with the case where only the insulation layer is machined.
If carbon dioxide laser light of fairly large energy is applied for machining as described above to reliably remove the conductor layer 4 for a printed wiring board as shown in FIG. 11A, while one-pulse laser light is being applied, the machining is advanced in the order of FIGS. 11B, 11C, 11D, and 11E and as shown in 11E, the conductor layer 4 on the laser light incidence side is projected into a hole, the hole shape becomes a middle swell shape, or damage to an inner conductor layer 5 occurs; this is a problem. The reason why such a problem occurs is that excessive heat is input to the insulation layer 1 because the energy of the carbon dioxide laser light is made large to machine the conductor layer 4.
The laser light incidence side where machining is performed, namely, the conductor layer 4 on the surface reflects the carbon dioxide laser light remarkably largely and the heat input amount and the thermal diffusion direction after heat input do not become stable and thus the roundness of a machined hole easily worsens and there is a problem of how to enhance the roundness of a machined hole.